1. Field
The present invention relates to a method of welding dissimilar metal materials by a friction stir welding method, and a welded body of dissimilar metal materials obtained by such welding method.
2. Related Art
Dissimilar metal materials, particularly, such as a steel material and an aluminum material, are generally welded by a melt welding method or mechanical bonding method such as using a rivet.
However, in the melt welding method, since the heat input amount is large there causes such a problem as that a brittle intermetallic compound (Fe2Al5, FeAl3 and the like) is formed in a boundary between the steel material and the aluminum material, resulting in lowering in the welding strength. In addition, in the mechanical welding method using rivets, bolts or the like, an additional member or material such as a rivet is needed for the welding, which results in increased cost.
For this reason, in recent years, research has been progressed for welding a steel material and an aluminum material to each other by a friction stir welding method (FSW: Friction Stir Welding) in which materials to be welded are softened without melt-welding the materials to thereby fluidize the softened materials and weld the materials to each other in a solid state. This friction stir welding method uses an FSW tool which is manufactured from a general tool steel, and brings the FSW tool into contact with only the aluminum material to perform the friction-stir-welding between the steel material and the aluminum material (see, for example, Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2005-34879; Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2006-239720).
For example, in the friction stir welding method described in Patent Document 1, a steel material of which the surface to be welded is covered with an antioxidant film (plated Zn film) and an aluminum material is overlapped with each other, the FSW tool is pressed to and inserted into the aluminum material while rotating the same, softening the aluminum material and the plated Zn film are then softened by applying a frictional heat to plastically fluidize the softened materials, the Zn film is removed to expose a newly-formed surface on the surface of the steel material, and the plastically fluidized aluminum material and the newly-formed surface of the steel material are finally welded to each other to realize a solid-state welding.
However, in the friction stir welding method performed without contacting the FSW tool to the steel material but only contacting to the aluminum material, the steel material and the aluminum material are not sufficiently stirred, and accordingly a high welding strength cannot be obtained. In other words, in a case where the FSW tool is brought into contact with the aluminum material and is inserted thereinto, when the temperature of the aluminum material rises to a temperature just below the melting point thereof, the aluminum material is softened and plastically fluidized to lower the friction strength with respect to the FSW tool. Therefore, the heat (frictional heat) is not generated any more, and the temperature cannot be raised to a temperature at which the steel material can be plastically fluidized, and thus, only the aluminum material is stirred, and the steel material and the aluminum material cannot be sufficiently stirred, and it becomes difficult to obtain a high bonding strength.
Furthermore, in a case where the FSW tool manufactured from a general tool steel is brought into contact with the steel material while being rotated in accordance with a conventional technique, the FSW tool may be worn or damaged by the adhesion of the steel material to the FSW tool.